what is KH?

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what is KH?
i read it was somthing to do with plants? is this wrong?
thanks...

Carbonate hardness or temporary hardness. Measures the buffering capacity or the ability to absorb and neutralize added acid without major changes to pH. Think of buffering capacity as a big sponge, the higher the buffering, the bigger the sponge. How much buffering does your tank need? The higher the kH (the bigger the sponge), the more resistant to pH changes your water will be. A tank's kH should be high enough to prevent large pH swings over time. If your kH is below roughly 4.5 OdH, you should pay special attention to your tank's pH (e.g., testing periodically) until you get a feel for how stable the pH is.
Buffering is both good and bad. On the good side, the nitrogen cycle in our tanks produces nitric acid (nitrate). If we donâ€™t have buffering (kH), the pH will drop over time. Sufficient buffering will keep the Ph stable. On the bad side, hard water almost always has a large buffering capacity and if the pH is to high for your fish, this large buffering capacity will make it more difficult to lower the pH.
Buffering is sometimes referred to as "alkalinity" but should not be confused with "alkaline". Alkalinity refers to buffering and alkaline refers to a solution that is base rather than acid (pH).

KH is carbonate hardness, temporary hardness, buffering capacity or alkalinity. This is a measure of the amount of buffering minerals in the water (mostly carbonates), that resist acidification of the water (it is basically the lime scale you see in the kettle).

The higher the KH, the more resistant your water will be when it comes to pH changes. Basically, if you were trying to use CO2, your KH should be no lower than 4dH otherwise, you have to monitor your pH carefully as low KH often results to pH crash. Your KH should measure higher than 4dH so large pH swings can be avoided.

To clarify further, KH is basically a pH buffer. The higher your KH, the more stable your pH will be and the more difficult to adjust your pH. If your KH is lower than 4, pH swings are likely to happen. Adding sodium bicarbonate will increase your KH therefore stabilizing the pH and preventing it from decreasing in all of a sudden which could result to pH crash killing the fish. If you use RO water, its KH is basically non-existent or zero, so pH can change dramatically with the use of RO water.

GH is general hardness or permanent hardness and is a mixture of calcium and magnesium salts. GH is independent to KH and pH.

There's one thing that bothers me. CO2 can reduce KH according to the site Eddie linked into? I don't think that would be true. If we have the KH below 4.5 dKH and we inject CO2, obviously the pH will crash but not the KH which serves as a buffering capacity.
I encountered this one in hopes of finding out if CO2 can really reduce the KH.

Quote:

CO2 does not lower KH. Because H2CO3 (carbonic acid), is an acid, it is assumed that it will lower the KH. This is not true as only the chemical species change which provide alkalinity.

H2CO3 --> H+ + HCO3-
HCO3- --> H+ + CO3--

The pH will change, but the KH remains the same. The net change is zero.

KH drops when a acid with a pKa (dissolution constant) greater than carbonic acid is added to the water from fish metabolites, bacteria, decay, etc. This lowers the KH as it exhausts the ability of the primary 'buffers', e.g. carbonate, bicarbonate to accept any more protons....(H+), called the carbonic acid equivalence point.

Edit: It seems the site was wrong to say that the CO2 can lower KH. KH is a buffering capacity and if a pH crash occurs, only the pH would reduce but not the KH which will stay the same.

The other possible mistake:KH can be increased by increasing surface agitation.
~Again KH can be increased by addition of sodium bicarbonate but aeration will not obviously produce sodium bicarbonate. So this is a mistake, isn't it?

There's one thing that bothers me. CO2 can reduce KH according to the site Eddie linked into? I don't think that would be true. If we have the KH below 4.5 dKH and we inject CO2, obviously the pH will crash but not the KH which serves as a buffering capacity.
I encountered this one in hopes of finding out if CO2 can really reduce the KH.

Quote:

CO2 does not lower KH. Because H2CO3 (carbonic acid), is an acid, it is assumed that it will lower the KH. This is not true as only the chemical species change which provide alkalinity.

H2CO3 --> H+ + HCO3-
HCO3- --> H+ + CO3--

The pH will change, but the KH remains the same. The net change is zero.

KH drops when a acid with a pKa (dissolution constant) greater than carbonic acid is added to the water from fish metabolites, bacteria, decay, etc. This lowers the KH as it exhausts the ability of the primary 'buffers', e.g. carbonate, bicarbonate to accept any more protons....(H+), called the carbonic acid equivalence point.